An optical transmission medium such as a plastic optical fiber has an outer shell and an optical transmission section of different reflective indices. The optical transmission medium transmits optical signals by reflecting incident light at interfaces between the outer shell and the optical transmission section.
Various suggestions have been made to produce the optical transmission medium having significantly reduced transmission loss. For instance, U.S. Patent Application Publication No. 2006/0173148 (corresponding to Japanese Patent Laid-Open Publication No. 2004-099652) suggests producing an optical member from a polymer composition in which two or more polymerizable compounds represented by certain general formulae are copolymerized in a certain mole ratio. The produced optical member has a very low transmission loss, and also prevents increase of the transmission loss caused by moist heat. Copolymers have an advantage in controlling the properties to be exhibited, for instance, optical properties such as a refractive index and the like, and thermal properties such as a glass transition temperature (Tg) compared to homopolymers which exhibit predetermined properties.
When a low molecular compound which does not contain a polymerizable component, that is, a non-polymerizable compound is added to the polymer to impart certain optical properties thereto, the non-polymerizable compound may affect other optical properties by diffusion or volatilization in the optical member. However, the copolymer exhibits such optical properties with a reduced amount of the non-polymerizable compound, or even without adding the non-polymerizable compound.
Japanese Patent Laid-Open Publication No. 06-297596 suggests an optical fiber constituted of a polymer mixture which contains a certain compound as a major component. In this optical fiber, a ratio of a structural unit is gradually changed in a diameter direction of a cross-section circle of the optical fiber. The above reference states that this optical fiber reduces the influence of infrared ray absorption caused by vibrations of C—H bonds, and initial properties of the optical fiber are not impaired with time.
However, in the case the optical transmission medium is produced from the composition disclosed in the above U.S. Patent Application Publication No. 2006/0173148, a loss of the light to be transmitted is caused by scattering of the light in the copolymer, which is hereinafter referred to as a scattering loss. The optical fiber of the Japanese Patent Laid-Open Publication No. 06-297596 reduces the transmission loss, and prevents the increase of the transmission loss caused by the moist heat. However, since the structure of this optical fiber has alkyl groups in a side chain, Tg of the optical fiber is low. As a result, under high temperature conditions, the transmission loss is increased, and a physical resistance is reduced to cause deformation. Accordingly, the optical fiber does not have sufficient practical utility.
An object of the present invention is to produce an optical transmission medium from a copolymer which resists high temperature and reduces the scattering loss.